The goal of this proposal is to investigate the role of the Survival of Motor Neuron (SMN) protein in neuronal apoptosis. We have generated a Sindbis virus model system to investigate the role of Survival of Motor Neuron (SMN) protein in neuronal apoptosis in vivo. Sindbis virus is a neuronotropic virus that causes a severe infection in young mice characterized by neuronal apoptosis. In this model system, the virus serves both as a cell death stimulus and as a vector expressing SMN specifically in CNS neurons. Preliminary studies reveal the first evidence that SMN modulates the susceptibility of neurons to programmed cell death in vivo and we propose that this function underlies the neuronal death seen in patients with Spinal Muscular Atrophy (SMA). Not only does SMN protein protect neurons from apoptotic death, but naturally occurring mutant SMN proteins found in SMA patients accelerate neuronal death, causing increased mortality in mice. Further data suggest a model in which SMN is cleaved by Caspase-6 in neurons undergoing apoptosis, creating a truncated protein that resembles a truncated SMN protein found in a patient with SMA, and that this truncated protein may have pro-death function. We now propose to further this analysis to determine the mechanisms that underlie the regulation of apoptosis by SMN. We will employ the Sindbis virus vector model system for in vivo gene delivery and induction of neuronal apoptosis. Domains of SMN critical for both pro- and anti-apoptotic functions will be investigated, and the identity of the SMN cleavage product created during apoptosis will be determined. The role of Caspase-6 in cleavage of SMN in vivo and whether suppression of this cleavage modulates the ability of SMN to protect neurons from apoptosis will be examined. Investigations will examine potential mechanisms that underlie the ability of SMN to modulate neuronal apoptosis, including whether SMN derivatives modulate differential splicing of selected pre-mRNA transcripts during apoptosis. Finally, a transgenic mouse approach will be employed to further the mechanistic exploration of SMN's ability to alter neuronal survival, and to determine the role of SMN in neural development and in mediating cellular responses to apoptotic stimuli.